Reflectometric insertion loss measurements for optical components

ABSTRACT

For determining an insertion loss (L DUT ) of an optical component ( 20 ) having one or more connections (C 1 , C 2 ), a connection insertion loss (L C1 , L C2 ) is determined for one or more of the connections (C 1 , C 2 ) by means a reflectometric measurement, and a total insertion loss (L total ) of the optical component ( 20 ) is determined together with its one or more connections (C 1 , C 2 ). The insertion loss (L DUT ) of the optical component ( 20 ) can then be determined from the determined total insertion loss (L total ) and each determined connection insertion loss (L C1 , L C2 ).

BACKGROUND OF THE INVENTION

The present invention relates to determining insertion loss of anoptical component.

Optical components are used in advanced optical networks and normallyundergo comprehensive tests during manufacturing. Components which arelikely to be coupled in concatenation connection in a large number (forexample in submarine applications) have the particular requirements thattheir insertion loss has to be characterized with high accuracy, such as10 mdB or even better. Those components are typically pigtailed, i.e.provided with an optical fiber at the input and/or output for opticallycoupling to the components, and need to be connected to a testinginstrument for characterization.

The problem, however, arises that the uncertainty introduced in ameasurement if a fiber optic connector or a bare fiber connection isbeing used might by far exceed the required accuracy for thecharacterization. Typical solutions for addressing this problem areeither to simply adapt the specification of the optical component to thelimited measuring accuracy, or to minimize the insertion loss resultingfrom optical connections to optical component. In the former case, thespecifications of the optical component usually contain theuncertainties of the connection to and from the optical component. Inthe latter case, the (e.g. pigtailed) fiber connections of the opticalcomponent are usually spliced in the lead fibers of the measurementinstrument. Such splicing connections can be reproduced with very lowlosses and uncertainties. This, however, has the disadvantage that thissplicing process takes additional time, and also some lengths of thepigtailed fiber connection of the optical component is being consumedfor each splicing.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedmeasurement of insertion loss of an optical component. The object issolved by the independent claims. Preferred embodiments are shown by thedependent claims.

According to the invention, a reflectometric measurement is appliedallowing determining a spatial resolution along the optical component tobe measured. As well known in the art, those reflectometric measurementsprovide an optical stimulus signal and measure, in return, a reflectedand/or backscattered signal in response to the stimulus signal (e.g.Raleigh and/or Raman scattering), as described e.g. the book ‘FiberOptic Test and Measurement’ by Derickson Dennis, 1998, ISBN 0-13-534330,in particular in chapter 11 thereof. Analyzing the received reflectedand/or backscattered signal (which shall be referred to in the followingas response signal) allows to conclude to the spatial characteristic ofthe insertion loss for the measured fiber path including all opticalcomponents coupled thereto.

For determining the insertion loss of a specific optical component to becharacterized, the invention provides a reflectometric measurement fordetermining the insertion loss of each connection of the opticalcomponent in the measuring path coupled to and accessible by thereflectometric measurement. The thus reflectometrically determinedinsertion loss(es) of the connection(s) of the optical component canthen be subtracted from a determined total insertion loss of the opticaldevice including the connection(s), thus resulting in an actualinsertion loss of the optical component to be measured, excluding theconnection(s).

While the total insertion loss of the optical component including itsconnection(s) might also be determined using the same or a differentreflectometric measurement, the measurement accuracy can besignificantly improved by determining the total insertion loss by meansof a transmission measurement. The transmission measurement determinesthe total insertion loss by evaluating the ratio of optical signalintensities before and after passing the optical device including itsconnection(s).

While the reflectometric measurement usually only requires a measurementfrom one side of the optical component, the transmission measurementrequires to measure from both sides of the optical component, i.e. froman input and an output of the optical component. Transmissionmeasurements, however, can be provided with higher accuracy thanderiving the transmission loss from a reflectometric measurement, sincethe power levels are generally much higher and therefore thesignal-to-noise ratio much lower in case of transmission. However,transmission measurements can only be provided in total for the opticalcomponents together with its connections and do not allow individuallyanalyzing the insertion loss at the connections of the opticalcomponents to be measured. Combining the reflectometric and transmissionmeasurement in a way that the reflectometrically determined insertionloss(es) of the connection(s) of the optical component is/are subtractedfrom the total loss of the optical component determined by means of thetransmission measurement thus allows to determine with high accuracy theactual insertion loss of the optical component.

In a preferred embodiment, the insertion loss of the optical componentis to be measured for a plurality of different wavelengths, as disclosede.g. in EP-A-872721 by the same applicant. Generally speaking, theinsertion loss of the connections of the optical component is normallyrelatively independent of the wavelengths in contrast to the actualinsertion loss of the optical component. Therefore, it is normallysufficient to determine the insertion losses of the connections of theoptical component for one wavelengths representative for a range ofdifferent wavelengths, and using those determined representativeinsertion losses for determining the actual insertion loss of theoptical component for that range of wavelengths. This allows tosignificantly reducing the calculation effort without jeopardizing theimproved accuracy as received by applying the insertion lossdetermination in accordance with the present invention.

The spatially resolved measurement can be provided by any kind ofreflectometric measurement as known in the art, in particular byapplying optical time domain reflectometry (OTDR), optical frequencydomain reflectometry (OFDR), frequency modulated continuos wavereflectometry (FMCW), or optical coherence domain reflectometry (OCDR).The latter is disclosed e.g. by E. Brinkmeyer and R. Ulrich,High-resolution OCDR in dispersive waveguides, Electronics Letters 26,pp. 413–414 (1990).

Typical optical components to be tested in accordance with the presentinvention can be fiber gratings, circulator, isolators, or otherintegrated-optical components. Those optical components are usuallypigtailed and provide as connections bare fiber connections and/or fiberoptic connectors. The invention thus allows reducing or eliminatinguncertainty that is introduced by the connections of an opticalcomponent to be measured, such as bare fiber connections and/or fiberoptic connectors.

It is clear that the invention can be partly or entirely embodied orsupported by one or more suitable software programs, which can be storedon or otherwise provided by any kind of data carrier, and which might beexecuted in or by any suitable data processing unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and many of the attendant advantages of the presentinvention will be readily appreciated and become better understood byreference to the following detailed description when considering inconnection with the accompanied drawings. Features that aresubstantially or functionally equal or similar will be referred to withthe same reference sign(s).

FIG. 1 illustrates the insertion loss measurement according to thepresent invention, whereby FIG. 1A shows an example of a measurementsetup, and FIG. 1B depicts an example of a measuring result receivedfrom a reflectometric measurement.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1A, a measurement instrument 10 (here: an optical time domainreflectometer—OTDR) is optically coupled to an optical component as adevice under test (DUT) 20. The DUT 20 is pigtailed, i.e. provided witha fiber 30 and a fiber 40, each for optically coupling to the deviceunder test 20. The measurement instrument 10 is coupled from a Port 1via a fiber 50 to the fiber 30, whereby a connection C1 is provided atthe coupling point between the fiber 50 to the fiber 30. The connectionC1 thus represents any kind of optical connection from the side of theDUT 20 towards the fiber 50 of the measurement instrument 10, such as aconnector provided at the end of the fiber 30 or a spliced fiberconnection between the fibers 30 and 50. Accordingly, the measurementinstrument 10 is coupled from a Port 2 via a fiber 60 to the fiber 40,whereby a connection C2 is provided at the coupling point between thefibers 60 and 40. The connection C2 thus represents any kind of opticalconnection from the side of the DUT 20 towards the fiber 60 of themeasurement instrument 10, such as a connector provided at the end ofthe fiber 40 or a spliced fiber connection between the fibers 40 and 60.

The DUT 20 together with the fibers 30 and 40 and the connections C1 andC2 shall represent a total measuring device 70, as it will be ‘seen’from outside of the measuring device 70 as being inserted between thefibers 50 and 60.

The measurement instrument 10 provides a reflectometric measurement ofthe measuring device 70 together with the thereto-connected fibers 50and 60, and might display the results of that measurement in a so-calledODTR-trace as depicted in FIG. 1B. The ODTR-trace of FIG. 1B shows thepower of the reflected and/or backscattered response signal versus adistance I relative from the measurement instrument 10. The backscattergenerally is due to Raleigh and/or Raman scattering in the fiber pathbetween Port 1 and Port 2. The OTDR-trace as shown in FIG. 1B assumesthat the measurement stimulus signal was launched from the Port 1.

An insertion loss L_(C1) of the connection C1 can be derived from theODTR-trace of FIG. 1B by determining the difference between the measuredresponse signal to the left and to the right of the connection C1.Accordingly, an insertion loss L_(C2) can be determined by thedifference in the response signal to the left and to the right of theconnection C2. Details on how to determine insertion losses at such kindof connections are disclosed e.g. in chapter 11 of the aforementionedbook ‘Fiber Optic Test and Measurement’.

An insertion loss L_(total) of the measuring device 70 can be determinedfrom the ODTR-trace as depicted in FIG. 1B by the difference in theresponse signal at the left side of the connection C1 and at the rightside of the connection C2. In a preferred embodiment, the insertion lossL_(total) is determined by a transmission measurement, whereby themeasurement instrument 10 determines an optical output power for themeasuring device 70 at (the right side of) the connection C2 and aninput power for the measuring device 70 at the (left side of) theconnection C1. The ratio between the determined outside power and thedetermined inside power thus represents the insertion loss L_(total).

It is clear that by keeping the fibers 50 and 60 sufficiently short,their insertion loss can be neglected, so that the optical power at thePort 1 represents the optical power at the connection C1, and theoptical power at the Port 2 represents the optical power at theconnection C2.

Preferably, in order to obtain high accuracy insertion loss measurement,the output power from Port 1 and the sensitivity of Port 2 have to beknown. This is typically done by referencing, i.e. connecting the fiber50 directly with the fiber 60 and storing the power value that has beenreceived in Port 2. In this reference measurement the connection betweenthe fibers 50 and 60 introduces new uncertainties. These also may beeliminated by using a reflectometric measurement scheme that allows tospatially resolve the losses.

It is to be understood that the insertion losses of the fibers 50, 30,40 and 60 are somehow exaggerated in FIG. 1B in order to illustrate theprincipal effect of those fibers. In most practical applications, thefibers 50, 30, 40 and 60 will be sufficiently short in length, so thattheir contribution to the insertion loss can be neglected.

For determining an insertion loss L_(DUT) (including the fibers 30 and40 but excluding the connections C1 and C2), the insertion losses L_(C1)and L_(C2) are subtracted from the insertion loss L_(total) of themeasuring device 70:L _(DUT) =L _(total) −L _(C1) −L ^(C2)with the losses given in logarithmic units. If the losses are given inlinear units, taking the ratio is required accordingly.

In case that the measuring device 70 is optically reciprocal, i.e. theresponse signal will return into the direction of the source of thestimulus signal, it is normally sufficient to provide the reflectometricmeasurement only from one side of the measuring device 70, e.g. onlyfrom Port 1. In case of a non-reciprocal measuring device 70,reflectometric measurements from both sides of the measuring devices 70are usually required, e.g. from Ports 1 and 2. It is thus clear, that incase of a reciprocal measuring device 70 and a determination of theinsertion loss L_(total) by means of the reflectometric measurement, themeasurement instrument 10 only needs to be coupled to one side of themeasuring device 70, e.g. only from Port 1. However, in case ofnon-reciprocal measuring device 70 and/or a determination of theinsertion loss L_(total) by means of a transmission measurement, bothsides of the measuring device 70 have to be connected to measuringinstrument 10 as depicted in FIG. 1A.

In case that the fibers 50 and 30 joined at connection C1 or the fibers40 and 60 joined at connection C2 have substantially differentbackscatter coefficients, it is also of advantage to providereflectometric measurements of the connection C1 and/or C2 from bothdirections (e.g. launching signals from the Port 1 for one and from thePort 2 for the other measurement). The relevant insertion loss for eachconnection C1/C2 can then be determined by the average of the insertionlosses determined by the two measurements from both directions, as it isknown from OTDR type measurements on fiber networks.

In a preferred embodiment, the measurement instrument 10 provides aninterferometric characterization of the measuring device 70, asdisclosed e.g. by David Sandel et.al. in “Optical Network Analysis andLongitudinal Structure Characterization of Fiber Bragg Grating”, Journalof Lightwave Technology, Vol 16, No 12, December 1998. This methodincludes measuring at various optical wavelengths the output signal ofan interferometer where the DUT is placed in one branch. Theinterferometer could be for example of Mach-Zehnder type for atransmittive DUT or of Michelson type for a reflective DUT. Examinationof the wavelength (or better: frequency) dependent output signal by forexample using a Fourier transformation allows a conversion of themeasurement signal similar to the OTDR-trace as depicted in FIG. 1B.This way, the measurement data available for characterization of themeasuring device 70 can also be used for characterizing the non-perfectconnections C1 and C2. Thus, the insertion losses of the connections C1and C2 can be obtained and treated separately from the properties of theDUT 20.

1. A method for determining an insertion loss (L_(DUT)) for a pluralityof measuring wavelengths within a range of wavelengths for an opticalcomponent (20) having one or more connections (C1, C2), comprising thesteps of: a) determining a connection insertion loss (L_(C1), L_(C2))for one or more of the connections (C1, C2) by means of a reflectometricmeasurement at a representative wavelength representative for theconnection insertion loss (L_(C1), L_(C2)) in the range of wavelengths,b) determining a total insertion loss (L_(total)) of the opticalcomponent (20) together with its one or more connections (C1, C2) for aplurality of measuring wavelengths within the range of wavelengths, c)determining, for each measuring wavelength, the insertion loss (L_(DUT))of the optical component (20) from the determined total insertion loss(L_(total)) at that measuring wavelength and from each determinedconnection insertion loss (L_(C1), L_(C2)) at the representativewavelength.
 2. The method of claim 1, further comprising determining theinsertion loss of the optical component by subtracting each determinedconnection insertion loss from the determined total insertion loss ifthe losses are given in logarithmic units, or by taking the ratio if thelosses are given in linear units.
 3. The method of claim 1, furthercomprising providing a transmission measurement for determining thetotal insertion loss of the optical component together with its one ormore connections.
 4. The method of claim 3, further comprisingevaluating the ratio of optical signal intensities before and afterpassing the optical component together with its one or more connectionsfor determining the total insertion loss.
 5. The method according toclaim 1, wherein determining the connection insertion loss (L_(C1),L_(C2)) for one or more of the connections (C1, C2) by means of thereflectometric measurement at a representative wavelength representativefor the connection insertion loss (L_(C1), L_(C2)) in the range ofwavelengths includes: providing the reflectometric measurement for ameasuring path including the optical component, and determining from thereflectometric measurement the connection insertion loss (L_(C1),L_(C2)) for each of the one or more connections within the measuringpath.
 6. The method according to claim 5, wherein determining from thereflectometric measurement the connection insertion loss (L_(C1),L_(C2)) for each of the one or more connections within the measuringpath includes: determining a first signal intensity in a close distancebefore one of the connections (C1, C2), determining a second signalintensity in a close distance after the one of the connections (C1, C2),and determining the connection insertion loss (L_(C1), L_(C2)) for theone of the connections (C1, C2) by processing the determined first andsecond signal intensities, preferably by subtracting the second signalintensity from the first signal intensity.
 7. The method according toclaim 1, wherein the reflectometric measurement is provided by applyingoptical time domain reflectometry (OTDR), optical frequency domainreflectometry (OFDR), frequency modulated continues wave reflectometry,or optical coherence domain reflectometry.
 8. A software program orproduct, preferably stored on a data carrier, for executing a method fordetermining an insertion loss for a plurality of measuring wavelengthswithin a range of wavelengths for an optical component having one ormore connections when the software program or product is run on a dataprocessing system such as a computer, the method comprising: determininga connection insertion loss for one or more of the connections by meansof a reflectometric measurement at a representative wavelengthrepresentative for the connection insertion loss in the range ofwavelengths; determining a total insertion loss of the optical componenttogether with its one or more connections for a plurality of measuringwavelengths within the range of wavelengths; and determining, for eachmeasuring wavelength, the insertion loss of the optical component fromthe determined total insertion loss at that measuring wavelength andfrom each determined connection insertion loss at the representativewavelength.
 9. A system for determining an insertion loss for aplurality of measuring wavelengths within a′ range of wavelengths for anoptical component having one or more connections, comprising: means fordetermining a connection insertion loss for one or more of theconnections by means of a reflectometric measurement at a representativewavelength representative for the connection insertion loss in the rangeof wavelengths, means for determining a total insertion loss of theoptical component together with its one or more connections for aplurality of measuring wavelengths within the range of wavelengths, andmeans for determining for each measuring wavelength, the insertion lossof the optical component from the determined total insertion loss atthat measuring wavelength and from each determined connection insertionloss at the representative wavelength.